1. Field of the Invention
This invention relates generally to surge suppression and more particularly, to monitoring the integrity of a neutral-to-ground suppression circuit.
2. Description of the Related Art
Surge suppressors are employed to prevent overvoltage conditions, typically by shunting excessive voltage transients caused by sources including lightning, inductive switching, electrostatic discharge, and unbalanced wye configurations. Surge suppressor devices often utilize Metal Oxide Varistors (MOV)s, Silicon Avalanche Diodes (SAD)s, selenium arrays, switched capacitors, switched resistors, or active components (e.g., Field Effect Transistor (FET), Bipolar Junction Transistor (BJT), Metal Oxide Silicon Controlled Thyristor (MCT), Insulated Gate Bipolar Transistor (IGBT)).
Surge suppressors generally must be as transparent as possible to system operation until needed to absorb excess energy. Thus, under normal system operation, surge suppressors exhibit an open, or high impedance state. Upon detection of intolerably high voltage (i.e. the clamping voltage of a surge suppressor), surge suppressors exhibit a low impedance state. Increased current is drawn into the surge suppressor due to its decreased impedance. The excessive energy shunted away from the load is received and partly or wholly absorbed by the surge suppressor. However, surge suppressors fail when they shunt too much energy. Therefore, surge suppressors are limited in the amount of energy which they may receive short of failure.
An MOV typically fails as a short circuit, causing a steep rise in current magnitude and ultimately an explosion unless the current path through the MOV is broken. Most often, a properly sized current fuse in series with an MOV breaks the current path through the MOV prior to explosion. In fact, the series fuse is usually sized to clear (i.e. blow) at a magnitude less than the MOV""s maximum current rating, sometimes preventing MOV failure in addition to preventing explosion.
Given the complexity of modern systems employing surge suppressors among many other components, it is desirable to obtain a clear indication when one component of the system fails. For instance, when an MOV in series with a current fuse is deployed to provide phase-to-neutral, phase-to-ground, or phase-to-phase protection, the phase voltage between the fuse link and the MOV will indicate a reduced voltage magnitude if the MOV fails and the fuse clears. Quite often, a visual indication is used to isolate this and other system failures. However, when an MOV and current fuse are deployed between ground and neutral, there is no phase voltage to monitor as an indication of surge suppressor failure. Instead, a neutral-to-ground fault circuit must sense and report surge suppressor failures.
One such circuit is disclosed in U.S. Pat. No. 5,432,667, xe2x80x9cNeutral-to-Ground Fault Sensing Circuit.xe2x80x9d Therein, inventors Rau and Bulson disclose a relatively complex and expensive circuit which deploys an MOV and two current fuses in series between neutral and ground wherein surge suppressor integrity is monitored at the center of the two fuses. The fuses are packed in sand within a dual fuse container. The theory of operation is that the fuses will clear when the MOV fails short. The theory intimates that both fuses will clear when one clears. However, there is no guarantee that both fuses will clear together, especially during a relatively low energy surge dampened by the thermal isolation provided by the sand. Further, the MOV failure will not be detected until and unless both fuses clear. Therefore, there is a need for an inexpensive and reliable indication of neutral-to-ground surge suppression failure.
This invention relates generally to surge suppression and more particularly, to monitoring the integrity of a neutral-to-ground suppression circuit. In accordance with the present invention, a neutral-to-ground fault monitoring circuit is disclosed, eliminating one or more disadvantages associated with the prior art. Regardless of the cause of failure (e.g., transient or continuous, or positive or negative overvoltage or excessive current) or the type of system (e.g., DC, single or at multi-phase AC), the disclosed invention indicates a neutral-to-ground fault caused by any one monitored component without requiring subsequent clearing of fuses or other device failures.
One embodiment of the fault monitor includes a suppression circuit and a monitoring circuit where the suppression circuit includes a first terminal, a second terminal, a surge suppressor, and a current fuse, wherein the surge suppressor and the current fuse are operatively connected between the first terminal and the second terminal and where the monitoring circuit indicates a fault upon sensing a loss of functionality of the surge suppressor or the current fuse.
Another embodiment of the fault monitor includes a suppression circuit and a monitoring circuit where the suppression circuit includes a first terminal, a second terminal, a first surge suppressor, and a second surge suppressor, wherein the first surge suppressor and the second surge suppressor are operatively connected between the first terminal and the second terminal and where the monitoring circuit indicates a fault upon sensing a loss of functionality of the first surge suppressor or the second surge suppressor.